1. Field of the Invention
The present invention relates to an ultrasonodiagnostic tomography apparatus and in particular to an electronic scanned ultrasonodiagnostic tomography apparatus.
2. Description of the Prior Art
In a prior art ultrasonodiagnostic apparatus for viewing an object to be inspected or specimen on the basis of a reflected echo of an ultrasound beam applied into the object, the number of transducers in the array transducer is selected by switching means and the aperture of transmitter is determined to be approximately equal with that of receiver. The prior art system is hereinafter referred to as the system having equal apertures for transmitter and receiver.
If the apertures of transmission and receiving are made larger in order to get higher resolution, the number of elements is increased because of the necessity for a nearly constant pitch of the array elements. This results in the drawback that it becomes impossible to realize the switching means.
On the other hand, if the element pitch is increased while holding the number of elements constant, the level of undesired sound signals is raised, resulting in the deterioration of image quality.
For obtaining favorable resolutions in all depth directions in the prior art system having equal apertures for transmission and receiving, it becomes necessary to use the so-called "multistage focusing (dynamic focusing)" wherein transmission is effected plural times while changing the transmission focus. When it is contrived to get higher resolution in this prior art system, the focal depth is rapidly decreased, and hence the number of stages is largely increased, the frame rate being largely lowered.